1. Industrial Useful Field
This invention relates to a manufacturing method for an electrode for use in a film type battery and a manufacturing method for an electrode-electrolyte composite.
2. Prior Art and its Problem
A conventional cathode for a film type battery has been manufactured as a sheet-shaped thin film by mixing conductor, binder and positive active material. In this method, however, a thickness of the thin film has been dependent on a particle size and a charging rate of the positive active material. It has been hard to obtain a sub-micron particle size and it has been difficult to increase the charging rate, so that it has been not easy to obtain a thin film having a thickness of under 10 microns inclusive.
Methods of utilizing lithium or lithium alloy for an anode have been proposed variously, however, it has been difficult to obtain a thin film of uniform thickness composed of lithium. In a method for manufacturing a thin film by extruding lithium from a nozzle for example, a lower limit of thickness of the thin film has been 0.03 mm, and the thickness has become not uniform under this thickness. Further, in a method for obtaining a thin film by rolling, a lower limit of thickness of the thin film has been 0.02 to 0.03 mm.
Carbon electrodes have been manufactured as a sheet-shaped thin film by mixing a carbon material and a binder. However, a thickness of the sheet has been dependent on a charging rate of carbon material, and it has been difficult to obtain a thin film electrode having a thickness of under 10 microns inclusive.
As described above, in the conventional method it has been difficult to obtain an anode or a cathode composed of an ultra thin film having an uniform thickness of under 10 microns inclusive, for example, with good productivity.
Methods for manufacturing the thin film electrode by means of a vacuum coating method, sputtering method etc. have been proposed. In these methods, however, a film forming rate is small so that the method is suitable for forming a film having a thickness of less than several hundred angstroms but not suitable for forming a film having a thickness of larger than that. On the other hand, in order to improve the film forming rate, a method wherein ultra fine particles of metal are directly carried in a gas flow to be deposited on a substrate at a high velocity has been proposed in Published Patent Application (KOKAI) No. 1-265450. However, this method has included such a disadvantage of an incontinuous and inefficient film forming process. Obtaining active materials of a multi-component system by this method has provided poor productivity because of the expense of fine-particles of various metals and the difficulty in mixing them uniformly. Moreover, it has been impossible to obtain active matrials of a multi-component system having an excellent property by this method.
In recent years, study and development of a secondary lithium battery are flourishing. Using metallic lithium for the anode in the secondary lithium battery will cause troubles from the standpoint of deterioration of performance and safety accompanied by dendrite formation at the time of charging. In order to overcome this problem, it has been proposed to use a carbon capable of occlusion and emission of lithium as the anode material. In a lithium battery utilizing a normal lithium intercalation active material as the cathode and a carbon electrode as the anode, it is necessary to previously dope the lithium in the carbon electrode. For this reason, the carbon electrode has required a troublesome process named as a doping process before assembling a package. In order to eliminate the previous doping process for the carbon electrode, it can be proposed that an active material having a structure reduced by lithium is synthesized to be used as the cathode active material. The manufacturing process can be simplified by this method. For example, LiMn.sub.2 O.sub.4 which is a spinel related lithium manganese oxide compound, is one which can be used as the cathode active material for a secondary lithium battery and can be synthesized easily. It is well known that a state of the cathode active material at the charging end will change up to Li.sub.2 Mn.sub.2 O.sub.4 when the battery is assembled by using the above material together with lithium related anode material and is discharged. Accordingly, if the spinel related manganese compound expressed by the chemical formula of Li.sub.2 Mn.sub.2 O.sub.4 can be synthesized, the secondary lithium battery can be composed by combining that compound and the carbon electrode which is not doped yet. In the present situation, however, it requires further various studies to chemically synthesize the lithium reductant called Li.sub.2 Mn.sub.2 O.sub.4, and a process named as previous discharging becomes necessary in order to synthesize the reductant electro-chemically. In order to eliminate the previous doping process for the carbon electrode, it can also be proposed to form a carbon electrode in which lithium has been doped previously.
On the other hand, it is desired to use a solid electrolyte, especially a solid polymer electrolyte, as an ionic conductive material for use in the film type battery. This is because the solid electrolyte has advantages of easiness in manufacturing, freeness from liquid leakage, and capability of manufacturing cells having voluntary shapes. When using the solid electrolyte, however, means for always maintaining a good adhesion state between the electrode and electrolyte, i.e. pressure welding of the two etc., will be required. With the film type battery, especially a battery provided with flexibility, it is difficult to apply a pressure from outside or inside as encountered in a prismatic battery or a spiral cylindrical battery, so that a serious trouble such as failure in adhesion between the electrode and electrolyte occurs.
In addition to the foregoing improvement in good adhesion between the electrode and electrolyte, the following points are required: [1] An effective contacting area between an electrode material and a packaging material which forms a current collector must be maintained in a good state. [2] In case of an organic electrolyte battery such as the lithium battery etc., it must be protected from influence of water content because a composite composed of the electrode and the electrolyte reacts with the water content to become an undesirable material for a battery.
Further, the following literatures are known as the prior art: [1] "Surface Technology" 8, 335, 1987, M. Oda, `Formation and Application of Ultra Fine particles by In-Gas Evaporation Method`, [2] "Applied Physics" 54, 687, 1985, C. Hayashi, `Gas Deposition of Ultra Fine Particles`, [3] "Japanese Journal of Applied Physics" 23, L910, 1984, S. Kashu, E. Fuchita, T. Manabe, C. Hayashi, `Deposition of Ultra Fine Particles Using a Gas Jet`.